Optical transmission fiber and process for producing the same

ABSTRACT

An optical transmission fiber comprising (1) a core of high refractive index composed of SiO 2  -based glass containing at least one of GeO 2 , As 2  O 3 , Sb 2  O 5 , SnO 2 , TiO 2 , PbO and Bi 2  O 3 , (2) a clad of low refractive index composed of SiO 2  -based glass containing at least one of F, F/B 2  O 3  and F/P 2  O 5 , and (3) an outermost jacket layer composed of SiO 2  and/or SiO 2  -based glass containing at least one of Al 2  O 3 , TiO 2 , ZrO 2  and HfO 2 .

This is a divisional of application Ser. No. 07/262,095, filed Oct. 19,1988 which is now U.S. Pat. No. 4,975,102, which is a continuation ofSer. No. 06/850,437 (now abandoned), filed Apr. 8, 1986, which is acontinuation of Ser. No. 06/617,865 (now abandoned), filed June 6, 1984,which is a continuation of Ser. No. 06/521,287 (now abandoned), filedAug. 8, 1983, which is a continuation of Ser. No. 06/200,351 (nowabandoned), filed Oct. 24, 1980.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical transmission fiber and to a processfor producing the same.

2. Description of the Prior Art

Remarkable advances have been made in the development of low lossoptical fiber for use in long-distance optical communications andcommercial production of such fiber has already begun. The fiber uses alight-emitting diode (LED) or laser diode (LD) as a light source and isdesigned to permit low-loss, broad-band transmission. For this purpose,the fiber comprises a core having a diameter up to 50 μm and a claddinghaving a refractive index that differs from that of the core by onlyabout 1%.

An optical fiber has several advantages, such as lack of susceptibilityto external induction, flexibility, light weight and large-capacitytransmission. To take full advantage of these advantages, applicationsof optical fibers are expanding to computers, instrumentation, controlsystems and graphic communications. In these applications, there is agreat demand for reducing the cost of manufacturing a transmissionsystem comprising a light source, optical fibers and a light receiver ora system comprising such a transmission system plus branches andmultiplexer circuits. Such systems use an inexpensive LED as a lightsource and achieve low system transmission loss by reducing thetransmission loss in the coupling of the light source and the opticalfiber. Needless to say, the fibers used in these systems must have highpractical strength, and an equally important requirement is that thefibers have low transmission loss. Therefore, there is a demand for anoptical fiber that does not have a transmission loss per kilometer ofmore than 10 dB, has a core diameter of about 100 μm and ischaracterized by a differential refractive index between the core andcladding of about 2%. Conventionally, an optical fiber that is supposedto meet the demand has been produced in the following manner: a powderof a core glass (P₂ O₅ -GeO₂ -SiO₂) is laid over the starting member byflame hydrolysis and outside vapor-phase oxidation (OVPO) process, andthe core glass is further overlaid with a powder of a cladding glass (B₂O₃ -SiO₂). After removing the starting member, the laminate is sinteredand collapsed to provide a preform which is spun into a fiber. The fiberproduced by such method is characterized by a core composed of a GeO₂-SiO₂ or GeO₂ -P₂ O₅ -SiO₂ glass having a diameter of 100 μm plus acladding composed of a B₂ O₃ -SiO₂ glass which, when having a diameterof 140 μm (core diameter+clad diameter), provides a refractive indexthat differs from that of the core by about 2%. It is common practice tocontrol the refractive indices of the core and cladding by mixing adopant such as GeO₂, B₂ O₃ or P₂ O₅ with the primary material SiO₂. Ahigh refractive index can be obtained using dopants such as P₂ O₅, GeO₂,Al₂ O₃ and TiO₂, but the only oxide available as a dopant for providinga low refractive index is B₂ O₃. A dopant has an effect on physicalproperties other than the refractive index of the primary material SiO₂; it reduces the viscosity and increases the thermal expansioncoefficient of pure SiO₂ glass to which it is added. Therefore, if thecore is doped with a large amount of GeO₂ or P₂ O₅ in an attempt toincrease the refractive index of the core, the viscosity of the coreglass is reduced and the thermal expansion coefficient of the core glassis increased. Since the difference in residual stress between the coreand cladding must be reduced to a minimum, the viscosity and thermalexpansion coefficient of the cladding must be controlled so as to offsetthe change resulting from the decreased viscosity and increased thermalexpansion coefficient of the core. Therefore, if the core is doped witha large amount of P₂ O₅ or GeO₂ to increase the refractive index of thecore, the cladding is likewise doped with a large quantity of B₂ O₃, butthen the reduced viscosity and increased thermal expansion coefficientprovide an optical fiber of low strength. In addition, the B₂ O₃ -SiO₂glass of which the outer cladding has low water resistance and greatstatic fatigue in a moist or humid atmosphere is experienced. Asmentioned above, B₂ O₃ is the only oxide dopant to provide a lowrefractive index. Thus, the cladding of all conventional optical fiberscontains B₂ O₃ and hence has low water resistance.

One may say this problem can be solved by covering the cladding with awater-resistant glass jacket, but this idea is impractical because thestate-of-art technology can seldom furnish a water-resistant glasscomposition whose viscosity and thermal expansion coefficient agree withthose of a soft core and cladding containing a large quantity of dopant.

SUMMARY OF THE INVENTION

The optical fiber of this invention is characterized by the following:the difference in refractive index between the core and cladding isabout 2%; the use of a dopant is minimized to retain the desiredhardness of the core and the cladding; a dopant such as P₂ O₅ or B₂ O₃that reduces the viscosity of glass is not used to provide a viscosityand thermal expansion coefficient as close as is possible to those ofpure SiO₂ glass thereby permitting the use of a jacket having a lowthermal expansion coefficient and which has a viscosity equal to orslightly less than that of the core or cladding during melt-spinning;and the overall result is increased strength of the fiber.

The fiber of this invention comprises:

(1) a section of SiO₂ -based glass containing at least one of GeO₂, As₂O₃, Sb₂ O₅, SnO₂, TiO₂, PbO and Bi₂ O₃ and having a high refractiveindex,

(2) a section of SiO₂ -based glass containing at least one of B₂ O₃, F,B₂ O₃ /F and P₂ O₅ /F and having a low refractive index, and

(3) an outermost layer of SiO₂ or SiO₂ -based glass containing at leastone of Al₂ O₃, TiO₂, ZrO₂ and HfO₂.

The fiber of this invention can be produced by the following procedure:forming a transparent glass rod composed of SiO₂ -based glass containingat least one of GeO₂, As₂ O₃, Sb₂ O₅, SnO₂, TiO₂, PbO and Bi₂ O₃ byeither the vapor-phase axial deposition (VAD) process or the OVPOprocess, overlaying the rod with molten SiO₂ -based glass doped with B₂O₃, SiO₂ -based glass doped with F, SiO₂ -based glass doped with F andB₂ O₃ and SiO₂ -based glass doped with F and P₂ O₅, covering thelaminate with molten glass composed of SiO₂ and/or Al₂ O₃ -SiO₂, TiO₂-SiO₂, ZrO₂ -SiO₂ or HfO₂ -SiO₂, and melt-spinning the resulting preformof the transparent glass rod to produce a fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows a cross section of the fiber according to thisinvention and the refractive index distribution chart of the fiber.

DETAILED DESCRIPTION OF THE INVENTION

By reference to the figure, a core 11 is composed of SiO₂ glass thatprovides adequate hardness to the core and which contains a dopant, suchas GeO₂, As₂ O₃, Sb₂ O₅, SnO₂, TiO₂, PbO or Bi₂ O₃ to provide a highrefractive index, for example, GeO₂ -SiO₂, As₂ O₃ -SiO₂, SnO₂ -SiO₂, Sb₂O₅ -SiO₂, TiO₂ -SiO₂, PbO-SiO₂ and Bi₂ O₃ -SiO₂ systems. Table 1 belowshows the melting point, viscosity and thermal expansion coefficient oftypical examples of the above systems. As is apparent from Table, thesesystems have a lower melting point, viscosity and thermal expansioncoefficient that other glass systems.

                                      TABLE 1                                     __________________________________________________________________________    SiO.sub.2   TiO.sub.2 --SiO.sub.2                                                                GeO.sub.2 --SiO.sub.2                                                                As.sub.2 O.sub.3 --SiO.sub.2                                                          SnO.sub.2 --SiO.sub.2                                                                Sb.sub.2 O.sub.5 --SiO.sub.2                                                          B.sub.2 O.sub.3 --SiO.sub                                                     .2     P.sub.2 O.sub.5                                                               --SiO.sub.2           __________________________________________________________________________    Amount of   10%    10%    10%     10%    10%     10%    10%                   Dopant                                                                        (wt %)                                                                        Refractive                                                                          1.458 1.490  1.473  --      1.475  1.478   1.457  1.464                 Index                                                                         Melting                                                                             1,713 1,690  1,650  1,550   1,670  1,600   1,410  1,400                 Point                                                                         (°C.)                                                                  Viscosity                                                                           Hard  Hard   Hard   Fairly  Hard   Fairly  Soft   Soft                                            hard           hard                                 Thermal                                                                             5.5 × 10.sup.-7                                                               1.2 × 10.sup.-7                                                                1.1 × 10.sup.-7                                                                --      --     --      1.3 × 10.sup.7                                                                 --                    Expansion                                                                     Coefficient                                                                   __________________________________________________________________________

There are two other dopants that can provide high refractive index,i.e., Al₂ O₃ and Ga₂ O₃, but if they are used in a large amount, theycrystallize and devitrification of the glass occurs.

A cladding 12 as shown in the figure is composed of SiO₂ doped with asmall amount of F and/or B₂ O₃. The glass composition of SiO₂ doped withF is such that part of the oxygen is replaced by F to provide arefractive index lower than that of pure SiO₂. However, the physicalconstants (i.e., melting point, viscosity and thermal expansioncoefficient) of the F-doped SiO₂ glass are almost the same as those ofpure SiO₂. On the other hand, SiO₂ doped with B₂ O₃ does not provide avery low refractive index, and instead, the melting point and viscosityof such a glass decrease and the thermal expansion coefficient of such aglass increases. Combining the two dopants provides a glass compositionhaving suitable physical constants. If desired, B₂ O₃ may be replaced byP₂ O₅. Since P₂ O₅ provides slightly increased refractive index, B₂ O₃is preferred as a dopant for providing a low refractive index, but oneadvantage of using P₂ O₅ is that a small amount of P₂ O₅ lowers thesoftening point of the glass to a greater degree than does B₂ O₃ andpreform-making and drawing operations are easy. The composition of thejacket glass is determined by the softness of the core and cladding. Ifthe composition of the core and cladding is close to that of pure SiO₂,the jacket may be composed of SiO₂ glass, but if the core or cladding israther soft, a dopant may be used to provide a jacket having a viscositysubstantially the same as that of the core or cladding. In this case,the refractive index of the jacket is selected at a value higher thanthat of the cladding, whereas the thermal expansion coefficient of thejacket is selected at a value slightly lower than that of the corecladding. This way the core, cladding and jacket provide matchedviscosity resistance upon melting, hence easy spinning is achieved. Thejacket glass becomes sufficiently soft upon melting to provide a smoothsurface, whereas the thermal expansion coefficient of the jacket glassis so small that when the laminate of the core, cladding and jacket isspun into a fiber, residual compressive stress develops on the surfaceand fiber is mechanically strong and less susceptible to fatiguefailure. As a further advantage, the difference in refractive indexbetween the core and cladding can be increased to a satisfactory degreeprovided that the refractive index of the jacket is not smaller thanthat of the cladding. An SiO₂ -based glass containing at least one ofAl₂ O₃, TiO₂, ZrO₂ and HfO₂ can be used with advantage as a compositionwhich is water-resistant and meets the above defined requirements ofviscosity, refractive index and thermal expansion coefficient. Thejacket may be provided with a light-absorbing function by, for example,incorporating a suitable amount of a transition metal such as Fe₂ O₃.The jacket may consist of an inner SiO₂ glass layer and an outer SiO₂-based glass layer containing at least one of Al₂ O₃, TiO₂, ZrO₂ andHfO₂. One example of the composition of a fiber providing the refractiveindex distribution shown in the figure is a core 11 composed of an SiO₂-glass doped with 10 wt % of GeO₂, a cladding 12 composed of an SiO₂-based glass doped with 3 wt % of F and a jacekt 13 composed of an SiO₂-based glass doped with 3 wt % of TiO₂.

Two specific methods of producing the fiber of this invention aredescribed hereunder, but it is to be understood that the fiber of thisinvention can be produced by other methods.

First, glass for the section of high refractive index is made asfollows: a halide such as SiCl₄ or GeCl₄, a hydride such as SiH₄ orGeH₄, or an organic compound such as Si(OC₂ H₅)₄ or Ge(OC₂ H₅)₄ issupplied as a vapor into an H₂ -O₂ flame or C_(m) H_(n) -O₂ flame tocause a reaction that forms finely divided GeO₂ -SiO₂ glass, which isgrown axially on a mandrel of silica glass (VAD process) or grownradially on a mandrel coated with carbon powder (OVPO process), therebyproviding finely divided glass. The glass particles made by the VADprocess are sintered as such and collapsed to form a transparent GeO₂-SiO₂ glass rod. The starting material is removed from the glassparticles made by the OVPO process before the particles are sintered andcollapsed to form a transparent GeO₂ -SiO₂ glass rod. The glass rod thusprepared is optionally ground and polished to provide a perfectcylinder, which is washed with hydrofluoric acid, hydrochloric acid,nitric acid, phosphoric acid or sulfuric acid before it is subjected tothe subsequent steps.

In the next step, H₂ O, HCl, Cl₂, HF or F₂ gas or a gaseousfluorine-containing compound such as SF₆, CF₄ or CCl₂ F₂ is suppliedinto a high-frequency plasma flame together with, optionally, helium oroxygen gas to form a plasma flame which is applied to the rotating glassrod to perform flame-polishing by moving the flame longitudinally overthe glass rod [plasma outer deposition (POD) process]. Subsequently, ifSiF₄, SF₆, CF₄ or CCl₂ F₂ is supplied with oxygen gas, a halide such asSiCl₄ or a hydride such as SiH₄ or an organic compound such as Si(OC₂H₅)₄ is supplied in a gaseous form into a high-frequency plasma flame asan SiO₂ forming material to form SiO₂ glass of a low refractive indexdoped with F. The resulting glass is grown in a molten state on theperiphery of the glass rod serving as the section of a high refractiveindex. SiO₂ glass doped with B₂ O₅ as well as F can be produced bysupplying both the gas serving as an SiO₂ forming material and the gasof a halide such as BCl₃ or BF₃, a hydride such as B₂ H₆, or an organiccompound such as Si(OCH₃)₃. To produce SiO₂ glass doped with P₂ O₅ aswell as F, both the gas serving as the SiO₂ forming material and the gasof a halide such as POCl₃ or PF₃, a hydride such as PH₃, or an organiccompound such as P(OCH₃)₅ can be supplied.

To form a glass serving as a jacket, the gas serving as the SiO₂ formingmaterial is supplied at a controlled rate and the gas serving as adopant, such as TiCl₄, AlCl₃, ZrCl₄ or HfCl₄, is supplied at acontrolled rate. In this case, the supply of the gaseous F-containingcompound is stopped, and if co-doping of F and B₂ O₃ was performed inthe previous stage, the supply of the dopant gases is also stopped.

The SiO₂ glass thus formed which contains a dopant such as TiO₂ is grownin a molten state on the periphery of the layer of low refractive index.In this manner, a preform comprising the core which is overlaid with thecladding and the jacket in that order is prepared. This method requirescareful temperature control to prevent the formation of seeds andblisters due to excessive heating of the core. If a seed or blister isfound to develop in the core which is being overlaid with a cladding, asmall amount of B₂ O₃ must be used as a codopant to produce molten glassat a relatively low temperature, and it is also necessary to use anincreased amount of a dopant in the material for a jacket to againproduce molten glass at a relatively low temperature.

Another method of producing the optical fiber of this invention is asfollows.

First, a silica glass tube is supplied with gases serving as materialfor glass (the same as described above) and heated externally (modifiedCVD process) or internally (plasma CVD process) to cause a reaction thatforms a deposit of either F-SiO₂, F-B₂ O₃ -SiO₂ or F-P₂ O₅ -SiO₂ glasson the internal wall of the tube. A glass rod serving as a section of ahigh refractive index having the same composition as defined above isinserted into the tube, which is set on a glass lathe and heatedexternally to collapse around the insert to form an integral compositeglass rod. If desired, a technique identical to that described above canbe used to coat the rod with SiO₂ glass containing either Al₂ O₃, TiO₂,ZrO₂ or HfO₂, thereby make a preform of a transparent glass rod. Thepreform is then melt-spun into a fiber.

One preferred embodiment of the process for producing the optical fiberof this invention is described below. Four preforms were prepared by theOVPO process, the VAD process and the process of this invention. Eachpreform was spun into a fiber, and immediately thereafter, the fiber wasprimed with a silicone resin in a thickness of 150 μm andextrusion-coated with nylon to provide a total diameter of 0.9 mm. Theconstruction, transmission characteristics and strength of each fiberproduced are shown in Table 2 below.

                                      TABLE 2                                     __________________________________________________________________________                                                    Tensile                                                                  Average                                                                            Strength                                                          λ = 0.85 μm                                                                Tensile                                                                            after                                                             Transmission                                                                         Strength                                                                           Immersion                     Sample               Size                                                                              Δn                                                                           Production                                                                          Loss   (kg/ in Water                      No. Glass            (μmφ)                                                                      (%)  Method                                                                              (dB/km)                                                                              filament)                                                                          (70° C. ×                                                        100 hr)                       __________________________________________________________________________    (1) Core: 32 wt % GeO.sub.2 --SiO.sub.2                                                            100 2.0  OVPO  6.0    5.5  x                                 Cladding:                                                                           20 wt % B.sub.2 O.sub.3 --SiO.sub.2                                                      140                                                      (2) Core: 28 wt % GeO.sub.2 /                                                                      100                                                                6 wt % P.sub.2 O.sub.5 --SiO.sub.2                                                           1.9  VAD   5.8    5.0  x                                 Cladding:                                                                           22 wt % B.sub.2 O.sub.3 --SiO.sub.2                                                      140                                                      (3) Core: 10 wt % GeO.sub.2 --SiO.sub.2                                                            100      VAD                                                 Cladding:                                                                           2.5 wt % F--SiO.sub.2                                                                    135 2.0  POD   4.0    7.0                                    Jacket:                                                                             5 wt % TiO.sub. 2 --SiO.sub.2                                                            140      POD                                             (4) Core: 12 wt % GeO.sub.2 --SiO.sub.2                                                             90                                                          Cladding:                                                                           2.5 wt % F/                                                                              100                                                                2 wt % P.sub.2 O.sub.5 --SiO.sub.2                                                           2.1  Modified                                                                            3.5    6.5                                    Jacket (a):                                                                         SiO.sub.2  140      CVD                                                 (b):  5 wt % TiO.sub.2 --SiO.sub.2                                                             145      POD                                             __________________________________________________________________________     "x" means poor, and "" means good.                                       

As Table 2 shows, the fiber of this invention suffers a transmissionloss per kilometer (λ=0.85 μm) of only 4.0 dB and its average tensilestrength is as high as 6.0 kg per filament. It is worth noting that thefiber exhibits significantly increased tensile strength in water.

The advantages of the process of this invention for producing an opticalfiber are summarized below.

(1) A fiber comprising a core of adequately large size, having arefractive index that greatly differs from that of the cladding, and thephysical properties of which match those of the cladding and the jacketis provided.

(2) The ratio of dopant to SiO₂ glass in the core is less than half thatconventionally required. The fiber produced has small loss due toRayleigh scattering, hence low transmission loss.

(3) The glass of which the jacket is made becomes soft at the spinningtemperature and provides a smooth surface. The resulting fiber hasresidual compressive stress left on the surface to provide highstrength.

(4) With the jacket made of SiO₂, ZrO₂ -SiO₂, Al₂ O₃ -SiO₂, TiO₂ -SiO₂or HfO₂ -SiO₂, the fiber has a highly water-resistant surface.

(5) The reduced use of a dopant such as GeO₂, B₂ O₃ or P₂ O₅ results ina corresponding cost reduction.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. An optical transmission fiber comprising:(1) acore section having a high refractive index comprising a GeO₂ -SiO₂glass or Sb₂ O₃ -SiO₂ glass, (2) a cladding section having a lowrefractive index consisting of a F-SiO₂ glass, a F-P₂ O₅ -SiO₂ glass, ora F-B₂ O₃ -SiO₂ glass, and (3) an outermost jacket layer comprising aglass selected from the group consisting of SiO₂, Al₂ O₃, Al₂ O₃, Al₂ O₃-SiO₂, TiO₂ -SiO₂, ZrO₂ -SiO₂, or HfO₂ -SiO₂.
 2. An optical transmissionfiber according to claim 1, wherein the fluorine containing-SiO₂ glassof the cladding section is produced by utilizing a high frequencyplasma.
 3. An optical transmission fiber as in claim 1, wherein therefractive index difference (Δn) between the core section and thecladding section is about 2%.
 4. An optical transmission fiber as inclaim 1, comprising:(1) a core section of 10 wt. % GeO₂ -SiO₂ glass, (2)a cladding section of 2.5 wt. % F-SiO₂ glass, and (3) an outermostjacket layer of 5 wt. % TiO₂ -SiO₂ glass.
 5. An optical transmissionfiber as in claim 4, wherein the refractive index difference (Δn)between the core section and the cladding section is about 2%.
 6. Anoptical transmission fiber as in claim 1 comprising:(1) a core sectionof 10 to 12 wt. % GeO₂ -SiO₂ glass, (2) a cladding section of 2.5 to 3wt. % F-SiO₂ glass, and (3) an outermost jacket layer of an SiO₂ -glassor a 3 to 5 wt. % TiO₂ -SiO₂ glass.
 7. An optical transmission fiber asin claim 6, wherein the refractive index difference (Δn) between thecore section and the cladding section is about 2%.
 8. An opticaltransmission fiber as in claim 1, wherein the jacket contains a dopantto provide a jacket with a viscosity substantially the same as that ofthe core or cladding.
 9. An optical transmission fiber as in claim 1,wherein the refractive index of the jacket is selected at a value higherthan that of the cladding, and the thermal expansion coefficient of thejacket is selected at a value lower than that of the core or cladding.10. An optical transmission fiber as in claim 1, wherein the jacket hasa low thermal expansion coefficient and a viscosity equal to or lessthan that of the core or cladding during melt-spinning process.
 11. Anoptical transmission fiber as in claim 1, wherein the jacket comprisesan inner SiO₂ glass layer and an outer SiO₂ -based glass layercontaining at least one of Al₂ O₃, TiO₂, ZrO₂ and HfO₂.
 12. An opticaltransmission fiber as in claim 1, wherein the jacket contains atransition metal to provide a light-absorbing function.
 13. An opticaltransmission fiber as in claim 1, wherein, except for refractive index,the physical properties of the core match those of the cladding and thejacket.
 14. An optical transmission fiber as in claim 1, wherein theratio of core diameter of the jacket is from 0.62 to 0.71.
 15. Anoptical transmission fiber according to claim 1, wherein the claddingsection is a F-P₂ O₅ -SiO₂ glass.
 16. An optical transmission fiberaccording to claim 1, wherein the cladding section is a F-B₂ O₃ -SiO₂glass.
 17. An optical transmission fiber according to claim 1, whereinthe cladding section is a F-SiO₂ glass.
 18. An optical transmissionfiber comprising:(1) a core section of 12 wt. % GeO₂ -SiO₂ glass havinga high refractive index, (2) a cladding section of 2.5 wt. % F-2 wt. %P₂ O₅ -SiO₂ glass having a low refractive index, and (3) an outermostjacket layer of SiO₂ or 5 wt. % TiO₂ -SiO₂.